Fiber optic Bragg gratings have been used for absolute strain and temperature sensing, cure monitoring, and have the potential for use as magnetic or electric field sensors. This requires the accurate tracking of narrow band wavelength shifts on the order of several nanometres. It has been proposed to use a tunable, narrow band light source, such as a DFB or DBR laser, to track the wavelength shifts. In this embodiment, timed signals of known wavelengths are used to interrogate the sensor. The shift in wavelength of the Bragg Grating is determined from the timing of a back-reflected signal.
Another approach has been the use of broadband source and a tunable detection system, such as achieved by use of a scanning monochrometer or a tunable acousto-optic filter which would scan the wavelength range of interest and determine the wavelength of the back-reflected signal. A further approach uses a spectrometer and a photodiode array.
Each of the above methods have limited time response and have not permitted such wavelength shifting sensors as Bragg gratings to have wide application or compete with conventional strain gauges. The practical use of optical guidedwave sensors, such as fibre optic sensors, in smart materials and structures, places severe requirements on the characteristics of any sensing system. The need to compete with, or even surpass in performance conventional sensors such as resistive foil or piezoelectric strain gauges or temperature sensors, amongst others, requires a system that provides high resolution, high speed sensing, and absolute sensing. Also of importance are low cost for large scale use and small size and ruggedness in order to service in potentially demanding aerospace environments. Of particular importance is the new field of "smart materials and structures", where fibre optic Bragg grating sensors would be embedded within composite material structures for various sensing applications such as; strain and temperature measurements, cure monitoring, load history tracking, acoustic emission detection and damage assessment.
The present invention satisfies a number of these requirements.